A longitudinal sandwich element including a core of binder fixed mineral wool lamellae assembled in a side by side relationship extending in the longitudinal direction of the sandwich element, the core having two major surfaces substantially parallel to each other, and two end surfaces substantially perpendicular to the major surfaces and parallel to each other and two side surfaces interconnecting the major surfaces and the end surfaces, where the mineral wool fibers are mainly perpendicular to the major surfaces, and at least one sheet is attached to at least one of the major sides of the core, and at least one of the two side surfaces is provided by one or more side lamellae, and where at least one of the side surfaces has been formed as a cut section into the one or more side lamella(s), the cut section fitting into a mating cut section of one or more other side lamella(s) of an adjacent sandwich element, and where each side lamella has a varying density and the cut section is in the region with the higher density.